docs: def is_feasible_i(self, x, i):
          """return True if value ``x`` is in the invertible domain of
          variable ``i``
  
          """
          lb = self._lb[self._index(i)]
          ub = self._ub[self._index(i)]
          al = self._al[self._index(i)]
          au = self._au[self._index(i)]
          return lb - al < x < ub + au